Glutamine auxotrophs in E. coli K12 have been isolated and characterized in order to enable the molecular cloning of some of the genes involved in the covalent modificaton of glutamine synthetase (GS). Two types of mutants have been characterized: (1) glutamine-requiring strains which lack any detectable glutamine synthetase activity, and (2) another isolated by M. Berberich which has an absolute requirement for glutamine, possesses approximately one third the wild-type GS activity. The GS in these latter strains is highly adenylylated under conditions in which the wild-type GS is unadenylylated. Preliminary genetic mapping by episomal complementation indicates that these mutations are complemented by the F'104 episome known to carry the gln D gene encoding the uridylyltransferase. Early results indicate that these mutants are affected in the gene encoding uridylyltransferase. Similar studies on the class of mutants that completely lack GS activity indicate that they are mutations affecting the structural gene gln A for GS. These mutants are complemented by F'133 and by pLC 28-36 (gln A plus ColEl) both strains which carry the gln A region of the chromosome.